The invention relates to composite tubing and more particularly to couplings for composite tubing.
Fiber reinforced composite materials are known in the art and desirable for various applications due to their light-weight, high strength characteristics. One application for composite materials is pipe that is made in tubular form with a fiber reinforced plastic material. Segments of the composite pipe have a significant use in the petroleum industry. However, in typical petroleum industry applications, the composite pipe will be subjected to high loads. Ideally, couplings which join the segments of the pipe should have the ability to withstand the same pressures and loads that are exerted on the pipe itself.
Composite pipe is commonly manufactured by winding or braiding reinforcing composite fibers that are impregnated with resin over a mandrel and/or an interior liner made of a thermoplastic or elastomeric material. The reinforcing fibers may be glass, carbon or other suitable material. The resin is later cured to form hard tubing. The fibers are typically in the form of filaments or xe2x80x9ctowsxe2x80x9d which are wound around the interior plastic liner or the mandrel to form the pipe.
Composite pipe is commonly manufactured in discrete lengths, usually up to about 30 feet in length, by the filament winding process where the mandrel is rotated within the filaments. Alternately, the tube may be manufactured as a continuous tube by either braiding or filament wrapping over a non-rotating winding mandrel which becomes an integral liner of the finished tube. FIG. 1 shows an example of a segment of composite pipe as it is being manufactured. The composite pipe 10 is formed as fibers 14 are wound around a plastic liner 12. FIG. 2A illustrates a type of filament winding machine 16 that is commonly used to manufacture composite pipe 10. The plastic liner 12 or a mandrel is drawn through several filament spool frames 20. These frames 20, as shown in FIG. 2B, rotate around the liner 12 while filament spools 18 unwind to extend fibers 14 which are then wound onto the liner 12 to form the composite pipe 10. When the desired length of the segment of pipe is reached, a connector must be added so that the segment can be attached to other segments of pipe. Consequently, it is advantageous for a connector for composite pipe to provide similar strength characteristics as the composite pipe when the two segments of pipe are attached together.
Prior art connectors for high-strength composite pipe for petroleum industry applications include the following types: (1) pinned joints that carry loads through radially oriented pins; (2) bonded joints that carry loads through the shear strength of an adhesive layer; (3) mechanically locked wedge-type joints that carry loads through a mechanical wedge; and (4) trap-type joints. The trap-type joint carries loads from the composite pipe to the connector by means of the composite fibers. The composite fibers are wound into grooves in the end of the end connector affixed to the composite pipe and are trapped in the groove by subsequently applied xe2x80x9choopxe2x80x9d or circumferential fiber windings. The trap-type joints are generally considered to provide the highest load-carrying capacity of the known composite connector types.
In the prior art, the fibers used with trap-type connectors are commonly xe2x80x9cwound intoxe2x80x9d the composite tube itself during the manufacturing process. The strongest prior art trap-type joint is most commonly provided by a discrete length filament winding. Alternatively, trap-type joints may be attached to an already cured composite pipe by applying additional fiber windings that are adhesively bonded to the cured pipe.
Prior art connectors include multiple grooves or traps for the stronger connections. Each fiber layer of the composite pipe typically carries the load to a selected trap for that particular layer. For example, where the pipe has five distinct fiber layers, the trap-type connector may have five separate traps or grooves (i.e., one for each layer). After each composite layer is completed, a hoop wrap is applied over the trap. The hoop wrap completely fills the trap while holding the fibers in place. The excess fiber extending beyond the trap may be trimmed at the distal end of the trap. The shape of the trap may be designed with various angles. This design allows the windings of each layer to lay against the bottom of the trap. This avoids xe2x80x9cbridgingxe2x80x9d the fibers across the trap as they are wound.
One aspect of the invention is a method for making a connection for composite pipe. The method includes attaching a connector having at least one trap to a segment of composite pipe comprising a plurality of filament fibers, winding the plurality of filament fibers across the end connector, wherein tension is continuously maintained on the filament fibers so that the filament fibers bridge across the at least one trap. The plurality of filament fibers that bridge across the at least one trap are compressed, and a binder interspersed in the filament fibers is then cured while tension on the fibers is maintained. In one embodiment, the connector has a plurality of traps. In one embodiment, the fibers in each trap are wrapped with a hoop wrap. In one embodiment, the hoop wrap in each trap has a modulus related to the position of the trap with respect to the end of the connector. In one embodiment, a flank angle of each trap is related to the position of the trap with respect to the end of the connector. In one embodiment, the width of each trap is related to the position of the trap with respect to the end of the connector. In one embodiment, the depth of the trap is related to the position of each trap with respect to the end of the connector.
Other aspects of the invention will be apparent from the description which follows.